This invention relates to fuses, and more particularly to current limiting fuses having auxiliary fusible elements.
Current limiting fuses of the type discussed herein conventionally include main and auxiliary fusible elements, each wound in helical fashion along an insulative core or the like support member. The core and fusible elements are embedded in a granular inert material of high dielectric strength, such as sand or finely divided quartz. The fusible elements usually take the form of one or more thin conductive strips or wires of silver, wound on the supporting core, which is made of high temperature resistant insulating material. The main and auxiliary fusible elements may each include one or more such conductive strips or wires, with the auxiliary fusible element being spaced apart from the main fusible element to preclude formation of electrical arcs therebetween under fault current conditions of low magnitude. The auxiliary fusible element is separated at its ends from the main fusible element usually by air gaps to produce multiple arc regions or burn back areas in the main fusible element under minimum current operation, thereby more effectively breaking the circuit through the fuse.
A current limiting fuse of the above-mentioned type is shown in U.S. Pat. No. 3,243,552, dated Mar. 29, 1966, to H. W. Mikulecky, assigned to the same assignee as the instant invention. The air gaps disclosed in that patent were formed by a pair of spaced-apart metallic terminals. Another U.S. Pat. No. 3,755,769, dated Aug. 28, 1973, also assigned to the same assignee as the subject invention, provides a more precise control over initiation of arcing action between the main and auxiliary fusible elements through the use of special porous tape members in place of the air gaps.
While the aforementioned arc initiating means provide acceptable control over the arcing action between the main and auxiliary fusible elements of a current limiting fuse of the above described type, it would be desirable if still more precise means could be provided for reliably initiating the formation of an electrical arc to the auxiliary fusible element for the introduction of the auxiliary fusible element into the fuse circuit. It would also be desirable to accurately control more precisely the time at which the auxiliary fusible element is introduced into the fuse circuit during the interruption cycle.